The present invention relates to the field of centrifugal compressors.
The invention relates more particularly to a centrifugal compressor impeller having a web and blades secured to the web on a front face of the web, each having a leading edge and a trailing edge, and the invention also relates to a centrifugal compressor including such an impeller, and to a turbine engine including such a centrifugal compressor. In this context, the term “turbine engine” designates machines such as, for example: straight-flow or bypass turbojets, turboprops, turboshaft engines, and/or turbocompressors.
In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of fluid through the compressor. The terms “front”, “rear”, “axial”, and “radial” are defined relative to the axis of rotation of the impeller.
A centrifugal compressor normally has a stationary portion and a rotary portion referred to as an “impeller” and carrying the rotary blades of the compressor. In operation, the impeller typically rotates at a high speed. It is therefore subjected to centrifugal stresses.
The shape of a centrifugal compressor impeller is determined by the flow of fluid through the compressor. Typically, in such a centrifugal compressor, the fluid enters into the compressor in a direction that is substantially axial, i.e. parallel to the axis of rotation of the impeller. The flow passage and the rotary blades direct the fluid radially outwards in such a manner that the fluid leaves the impeller in a direction that is substantially orthogonal to the axis of rotation of the impeller. The blades therefore have leading edges that are substantially radial and trailing edges that are substantially axial, located further away from the axis of rotation of the impeller in the radial direction, and situated axially behind the leading edges.
The web secures the rotary blades together and secures them to the shaft of the compressor. For this purpose, each blade is secured to the web and is situated on a front face of the web. The web also serves to define the root face of the fluid flow passage through the impeller. The web is thus normally axisymmetric and curves progressively outwards in the axial direction. By virtue of the web and the blades having this shape, centrifugal acceleration generates a bending moment on the impeller tending to bend the periphery of the impeller forwards. This bending moment increases continuously going from the periphery of the impeller towards the connection between the web and the shaft of the compressor, and it makes it necessary to maintain large amounts of clearance when the compressor is operating at intermediate speeds, thereby penalizing the performance of the machine. In order to withstand this moment, proposals have typically been made to reinforce the web and the means for fastening the impeller to the rotary shaft. Nevertheless, reinforcing the rotary portions of the impeller of a compressor in this way leads to a very significant weight penalty, since weight that is added close to the air flow passage will also require an increase in the bulk of the impeller.
To overcome that drawback, U.S. Pat. No. 4,060,337 proposes eliminating a large portion of the impeller web and connecting the blades solely at the base and at the periphery. Nevertheless, that compressor suffers from a significant drop in the aerodynamic performance of the impeller as a result of flow from the pressure side to the suction side of each blade.
In British patent application GB 2 472 621 A, proposals are made to connect the impeller to the rotary shaft via two rims with an axial offset in order to restrict the presence of material on the impeller solely to its functional zones. US patent application US 2010/0098546 A1 proposes making the web of the impeller hollow in its periphery so that the peripheral weight of the impeller is limited and is positioned optimally, thereby enabling the compressor to be optimized. Nevertheless, the weight reductions that can be obtained in those two ways are penalized by the difficulty in fabricating the final single-piece part. German patent DE 906 975 proposes an impeller in which the web is further forward in the axial direction at its periphery than at an intermediate diameter of the impeller. Nevertheless, that web also requires a reinforcing disk to be fastened to the blade tips, in order to restrict deformation of the periphery of the impeller in an axial direction, which may be difficult to adapt to an existing compressor or to an aeroengine, where restricting weight is a major priority. US patent application US 2007/0077147 A1 and British patent GB 553 747 show other impellers with webs that are advanced at the periphery, but that are nevertheless not proposed for solving the problem of axial deformation of the impeller at high speeds.